Balance in the feedback loop components of the mammalian cochlear amplifier

Abstract

The objective of this study is to identify the components of the cochlear amplifier feedback loop, viz., the system transfer function and the feedback gain. For maximum positive feedback amplification, loop gain, which is the multiplication product of these transfer functions, would approach unity. Near-unity loop gain requires that the system transfer function and the feedback gain be almost reciprocally matched. Whether such a matched condition exists remains to be shown and would depend on the mechanism of the cochlear amplifier. For the outer hair cell (OHC) electromotility hypothesis, a local lumped model shows that around the characteristic frequency, the feedback gain, which is proportional to the OHC transmembrane potential, is almost reciprocally matched with the system transfer function. This finding emphasizes that the low-pass filtering of the OHC transmembrane potential is not a challenge; instead, it is necessary for positive feedback amplification in the cochlea. Furthermore, such a balance of the feedback loop components exists over the tonotopic axis despite tonotopic parameter variations. These observations made from the local model are shown to remain valid despite global coupling using the global coupled mechanical–electrical–acoustic model of the cochlea.